Formation properties while drilling or in a freshly drilled hole are measured to predict the presence of oil, gas and water in the formation. These formation properties may be logged with wireline tools, logging while drilling (LWD) tools, or measurement while drilling (MWD) tools. Measurements are usually performed open hole, with the wellbore containing fluid at a hydrostatic pressure in excess of the reservoir pressure, so the formation is not producing any fluid into the wellbore. Therefore in this case wellbore fluid measurements generally do not contain information about fluids in the formation.
These openhole measurements of the formation properties, which may be considered static, because there is no formation fluid movement, may be used to infer the dynamic properties of the formation when the well is produced. When the well is produced, the pressure in the wellbore is less than the reservoir pressure. This condition may be achieved while drilling by way of a new technique called Under Balanced Drilling, or UBD. In this case the well is being drilled and produced simultaneously, so in this measurements of the fluid in the wellbore may contain information about the fluids which are being produced from the formation.
When drilling underbalanced, large quantities of drilling fluids are pumped through the drill string into the wellbore while the wellbore is being drilled. The drilling fluids help cool the cutting surfaces of the drill bits and help carry out the earth cuttings from the bottom of the wellbore when they flow up the annulus to the surface. To ensure that formation fluids flow into the wellbore during this underbalanced drilling process, the drilling fluids are pumped under a pressure that is slightly lower than the expected formation pressure. The lower hydraulic pressure of the drilling fluids may result in a substantial gain of fluid into the wellbore from the formation when a permeable and high pressure zone of the earth formation is encountered. Detection of such fluid production may be used to evaluate the inflow potential of the well, and to modify this inflow by making corresponding changes to the completion of the well. Cumulative fluid flow production from the formation may be detected on the surface. However, for determining the precise depth of each individual contribution to this fluid production, a means of detecting volumetric flows in the wellbore annulus near the drill bit as the well is being drilled is desirable.
Time-of-flight measurement of activated slugs of fluid have been used in the prior art in connection with the Water Flow Log (WFL). In the WFL service, a slim tool is lowered into a producing well, a slug of wellbore fluid is activated and then timed over a relatively long duration to determine the flow rate. In this process, an activation source such as a Pulse Neutron Genrator (PNG) is normally off, and is activated only very briefly to periodically tag a slug of fluid with a neutron burst.
It would be desirable to have methods and apparatus in connection to underbalanced drilling for determining various parameters at a given depth in the wellbore. It is particularly desirable to determine the depth of water producing fractures which are not discernable from resistivity logs. By determining these depths, one may design adequate completion in order to block the flow of undesirable water, for example by altering the producing pipe that is later installed in the well.